kW to Joules Converter
Kilowatts measure power, joules measure energy — you need the time to go from one to the other. Enter power and duration to get energy in joules, kJ, kWh, BTU and more, or run it in reverse.
Kilowatts to joules
Enter the power draw and how long it runs. Energy = power × time.
Rough equivalent only — flat tariff, no taxes or demand charges. For real bills see our energy cost calculator.
Joules to kilowatts
Reverse mode: given an amount of energy released over a time period, find the average power. P(kW) = E(J) ÷ (1000 × t(s)).
kW to joules reference table
Energy produced by common power ratings over 1 second, 1 minute and 1 hour. Note how 1 kW for 1 second is exactly 1000 J — and for 1 hour, 3.6 million.
| Power | 1 second | 1 minute | 1 hour |
|---|---|---|---|
| 0.1 kW | 100 J | 6,000 J | 360,000 J |
| 0.5 kW | 500 J | 30,000 J | 1,800,000 J |
| 1 kW | 1,000 J | 60,000 J | 3,600,000 J |
| 2 kW | 2,000 J | 120,000 J | 7,200,000 J |
| 5 kW | 5,000 J | 300,000 J | 18,000,000 J |
| 10 kW | 10,000 J | 600,000 J | 36,000,000 J |
Worked examples (hand-verified)
Each example is computed step by step and cross-checked against an independent Python oracle.
This is the question behind “how many joules does a 2 kW heater use per hour”: ✓ 7,200,000 J. At $0.15/kWh that hour costs $0.30.
Three minutes of reheating moves about a fifth of a megajoule into your food: ✓ 216,000 J.
The average power behind that energy release: ✓ 0.833 kW. Try it in the “Joules → kW” tab.
kWh to joules: the 3.6 million rule
One kilowatt-hour is the energy of 1 kW drawn for 1 hour. Since 1 W = 1 J/s:
1 kWh = 1000 W × 3600 s = 3,600,000 J
So a 2 kW appliance running 3 hours uses 6 kWh = 21,600,000 J. Utility meters and bills use kWh because joule counts get unwieldy — this page speaks both. For converting between energy units with no power or time involved (therm ↔ BTU, eV, tonnes of oil equivalent…), see our energy unit converter hub.
Methodology & unit definitions
The core physics
Power is the rate of energy transfer. The SI watt is defined as one joule per second (1 W = 1 J/s), so energy is simply power accumulated over time: E(J) = P(W) × t(s). With power in kilowatts the formula becomes E(J) = 1000 × P(kW) × t(s).
Unit factors used
- 1 kW = 1000 W; 1 MW = 1,000,000 W; 1 mechanical hp = 745.699872 W (exact definition).
- 1 kWh = 3,600,000 J exact; 1 Wh = 3600 J (one watt-second is one joule).
- 1 thermochemical calorie = 4.184 J exact (defined); 1 kcal = 4184 J.
- 1 BTU (International Table) = 1055.05585262 J, shown rounded to 1055.06 J.
- Results use full double precision internally; display rounds to at most 4 decimals with thousands separators.
Why thin converter pages get this wrong
Many single-formula pages treat “kW → joules” as a direct unit conversion. It is not: kilowatts and joules are different physical quantities. Any answer that omits the time period is meaningless — this page requires time in both directions and shows the formula with your numbers substituted, so every result is auditable.
Honest limitations
Scope boundary: which tool for which question
Converts between energy units directly — therm to BTU, electronvolts, tonnes of oil equivalent — with no power or time involved.
Open the hub →Turns usage into money: tariffs, tiers and bill estimates. This page computes the energy; that tool computes the bill.
Open cost tool →This page: “my device is rated X kW and runs Y long — how many joules is that?” — power × time → energy, with the time context built in, in both directions.
Frequently asked questions
What is the formula to convert kW to joules?
The energy E in joules equals 1000 times the power P in kilowatts times the time t in seconds: E(J) = 1000 × P(kW) × t(s). The 1000 converts kilowatts to watts, and multiplying watts (joules per second) by seconds gives joules. For example, 3 kW for 3 seconds gives 1000 × 3 × 3 = 9000 J.
Can you convert kW to joules directly?
No. Kilowatts measure power (the rate of energy use) while joules measure energy (the total amount). They are different quantities, so you cannot convert one to the other without knowing the time period. You can only calculate joules from kilowatts and seconds, using E(J) = 1000 × P(kW) × t(s). This calculator asks for the time for exactly that reason.
How many joules are in 1 kW?
It depends on the time: 1 kW running for 1 second produces 1000 joules; for 1 minute it produces 60,000 joules; for 1 hour it produces 3,600,000 joules (3.6 MJ). There is no single answer without a time period, which is why the question only makes sense as energy produced by 1 kW over a stated duration.
How many joules are in 1 kWh?
Exactly 3,600,000 joules (3.6 × 10⁶ J). Derivation: 1 kWh = 1000 W × 3600 s, and 1 W = 1 J/s, so 1000 × 3600 = 3,600,000 J. So a 2 kW heater running 3 hours uses 6 kWh, which equals 21,600,000 J.
How do you convert joules to kW?
Use P(kW) = E(J) ÷ (1000 × t(s)). Divide the energy in joules by 1000 times the time in seconds. For example, 90 joules released over 3 seconds means 90 ÷ (1000 × 3) = 0.03 kW. Switch this calculator to the “Joules → kW” tab to do it with any energy and time units.
How many joules does a 2 kW heater use per hour?
7,200,000 joules (7.2 MJ), which is also 2 kWh. Calculation: E = 1000 × 2 kW × 3600 s = 7,200,000 J. At an electricity price of $0.15 per kWh, that hour of heating costs about $0.30. Try the “Space heater” preset above to see the full breakdown.
What is the kW to kJ formula?
E(kJ) = P(kW) × t(s). Because 1 kJ = 1000 J, the 1000 in the joules formula cancels out: kilojoules simply equal kilowatts times seconds. So 3 kW for 3 seconds = 9 kJ, and 2 kW for 1 hour = 7200 kJ.
How do I calculate joules from kW and minutes?
Convert minutes to seconds first (multiply by 60), then apply E(J) = 1000 × P(kW) × t(s). Example: 1.2 kW for 3 minutes = 1.2 kW × 180 s × 1000 = 216,000 J. This calculator accepts minutes directly so you can skip the manual conversion.
Is a watt-second the same as a joule?
Yes. By definition 1 watt = 1 joule per second, so 1 watt sustained for 1 second delivers exactly 1 joule. “Watt-second” and “joule” are two names for the same SI unit of energy, just as this calculator’s formula shows.
How many joules are in 1 BTU?
1 BTU (International Table) = 1055.06 joules. The BTU is the energy needed to heat one pound of water by one degree Fahrenheit. For reference, 1 kWh equals about 3412.14 BTU. This calculator shows BTU alongside joules for every result.
What is the difference between kW and kWh?
kW (kilowatt) is power — how fast energy flows right now. kWh (kilowatt-hour) is energy — how much was used over time. A 2 kW heater draws 2 kW whenever it is on (power); running it for 3 hours consumes 6 kWh (energy). Utility bills charge for kWh, not kW.
Why does the time period matter so much?
Because energy is power accumulated over time. A 10 kW industrial motor running 30 seconds uses 300,000 J, but the same motor running all day uses 864,000,000 J — nearly 3000 times more. Any “kW to joules” answer that omits time is meaningless; beware of thin converter pages that hide this.
How accurate is this calculator?
The math is exact per SI definitions, but it assumes constant power over the whole period. Real appliances cycle on and off (a “2 kW” heater with a thermostat may average far less), motors have efficiency losses, and voltage fluctuations change actual draw. Treat results as the energy for the stated constant power — a solid estimate, not a meter reading.
Can I use this to estimate my electricity bill?
Roughly. Enter your appliance’s kW and usage time, read the kWh row, and multiply by your tariff — or type your price per kWh into the optional cost field. For real bills with tiered tariffs, demand charges, or taxes, use our dedicated energy cost calculator instead; this page gives the energy math, not billing logic.
How many joules per hour is 1 horsepower?
One mechanical horsepower sustained for one hour produces 2,684,519.5 joules (about 2.68 MJ), equal to 0.7457 kWh. Calculation: 745.699872 W × 3600 s = 2,684,519.5 J. Select “hp” as the power unit in the calculator to run this yourself.
How is this different from an energy unit converter?
This page answers “I have a device rated in kW running for some time — how much energy is that in joules?” with the time context built in, plus reverse mode and practical presets. Our separate energy unit converter hub converts between energy units directly (therm to BTU, eV, tonnes of oil equivalent, and more) without any power or time involved. Different questions, different tools.